Former world No.1 Serena Williams won't feature at this summer's Hopman Cup, but organisers remain optimistic Swiss star Roger Federer will be able to play at the mixed-teams tennis event.
Williams appeared at the past two Hopman Cups - with very mixed results.
She reached the final two years ago alongside John Isner, but the 22-time grand slam winner was forced to pull out of last summer's edition just minutes before her opening singles match because of a knee injury.
Williams, who turns 35 later this month, recently lost her No.1 world ranking to Angelique Kerber.
World No.33 Coco Vandeweghe will represent the US alongside Olympic gold medallist Jack Sock.
Two-time Wimbledon champion Petra Kvitova will make her third Hopman Cup appearance when she partners Adam Pavlasek for the Czech Republic.
Federer confirmed he would play at the Hopman Cup in July.
But just a week later, he ruled himself out of the Olympics and the rest of the year in order to rehabilitate his surgically-repaired knee.
Federer is still aiming to make his return at the January 1-7 Hopman Cup, but any setback could scupper those hopes.
Australian duo Nick Kyrgios and Daria Gavrilova have been invited back to defend their title, but they aren't locked in yet.
Tournament organisers will announce the teams for Great Britain, Germany, France, Spain, and Australia at a later date.
Kvitova, who won the Hopman Cup title in 2012, said she was excited to come back.
"It's a big honour for me to play under the Czech Republic," Kvitova said.
"Hopman Cup is something special as well. It is always something more you are playing for, not only for you, but for the country you love and I always love to represent my country."
Sock will enter the tournament as one of the world's form doubles players after taking home gold in the mixed doubles and bronze in the men's doubles in Rio.
2017 TEAMS ANNOUNCED SO FAR
Switzerland: Roger Federer and Belinda Bencic
Czech Republic: Adam Pavlasek and Petra Kvitova
US: Jack Sock and Coco Vandeweghe
